Due to energy cost and conservation, there is a demand for refrigerant motor-compressor units which have an output which can be varied in accordance with demand. To satisfy this demand, many different systems have been proposed. One such system involves the unloading of one or more cylinders in a multi-cylinder compressor or the varying of re-expansion volume for the purpose of varying the output of the compressor system. These systems tend to be relatively complex and the efficiency of the compressor system in the unloaded state is not optimum. Variable speed compressors have also been used, but they require expensive controls and also the speed control and motor-compressor efficiency present some efficiency issues at least when operating in a reduced output condition.
Compressor systems have also been developed which, in place of a single compressor large enough to carry the maximum load, include a plurality of smaller motor-compressors having a combined output equal to the required maximum. These multi-compressor systems include means for controlling the total system in such a manner as to selectively activate and deactivate less than all of the compressors when it is desired to vary the output. These multi-compressor units have good efficiency but they require complex hook-up plumbing, including means for dealing with lubricating oil management in order to ensure that all the oil remains equally distributed between each of the compressors.
Further development of the multi-compressor systems has included the incorporation of a plurality of standard motor compressor units in a common shell. The common shell maximizes the compactness of the system and provides a common oil sump for equal oil distribution, a common suction gas inlet and a common discharge gas outlet. These single shell multi-compressor units have proved to be acceptable in the marketplace but they tend to be relatively large and the means for controlling the total system is still somewhat complex.
The continued development of multi-compressor systems has been directed towards reducing the overall costs and the overall size as well as simplifying the control systems which dictate the output quantity of these systems.
The present invention provides the art with a dual compressor system with one compressor being located at opposite ends of a common drive shaft. A motor rotor is press fit to the center portion of the drive shaft and the motor rotor is disposed within a motor stator. Thus, both compressors are powered by the same motor. The control of the output of the dual compressor system is accomplished by a variable speed motor or by a pulsed width modulation (PWM) capacity control system incorporated into one or both of the two compressors. When incorporating a variable speed motor for capacity control, the capacity can be varied from 0% to 100%. When incorporating the PWM capacity control system into one of the compressors, the capacity can be varied from 50% to 100%. When incorporating the PWM capacity control system into both compressors, the capacity can be varied from 0% to 100%. The capacity of one or both of the compressors can be increased to approximately 120% of capacity using vapor injection to increase the range of the dual compressor system if desired.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.